First Law of Thermodynamics question

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a quantity of gas in a cylinder recieves 1600J of heat from a hot plate. At the same time 800J of work are done on the gas by outside forces pressing down on a piston. Calculate the change in thermal energy of the gas


Input heat Q = 1600J
The amount of work done on the gas is, W = 800J

now i'm not exactly sure how I'm supposed to set this up

do I use U = Q - W
or Q = W + U

so i either get 800J or 2400J for the answer for U

i know its a simple question but i think i'm just a bit confused conceptually. any help would be great :)
 

Answers and Replies

  • #2
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It should be [itex]\Delta U = Q + W[/itex], where Q is the heat input to the system and W is the work done on the system. In [itex]\Delta U = Q - W[/itex], W is the work done by the system.
 
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in10sivkid said:
a quantity of gas in a cylinder recieves 1600J of heat from a hot plate. At the same time 800J of work are done on the gas by outside forces pressing down on a piston. Calculate the change in thermal energy of the gas


Input heat Q = 1600J
The amount of work done on the gas is, W = 800J

now i'm not exactly sure how I'm supposed to set this up

do I use U = Q - W
or Q = W + U

so i either get 800J or 2400J for the answer for U

i know its a simple question but i think i'm just a bit confused conceptually. any help would be great :)
you are correct.
it's the first form:
ΔU = Q - W,
where ΔU is the change in internal energy, Q is the heat added to the system, and W is the work done by the system. of course, when work is done to the system (like your problem), W is negative, so that -W is positive, resulting in an increase in internal energy (like your problem):
ΔU = (1600) - (-800) = 2400 J
check here for more info:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/firlaw.html
 
Last edited:

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